Why did Sai Baba break his promise to attend Dev's Udyapan ceremony, which led Dev to write a disappointed letter to Jog?

The premise that Sai Baba broke his promise is a misunderstanding that Dev himself initially had. According to the account in Chapter 40, Baba did, in fact, attend the Udyapan ceremony, but he came in the guise of a Sanyasi accompanied by two other people. Dev failed to recognize him and, feeling disappointed, wrote to Jog complaining that Baba had not kept his word. Baba later clarified through Jog, stating, "He says I deceived him with My words of assurance; tell him, 'You did not recognize Me!'" Baba confirmed he fulfilled his promise precisely, explaining, "I said I would come with two others, and I went with two others," thereby proving his word is infallible even if his ways are mysterious.